Actuator materials and mechanisms that convert electrical, chemical, thermal, or photonic energy to mechanical energy have been sought for over a century. Nevertheless, humankind has had little success in replicating the wondrous properties of natural muscle, which has meant that the most advanced prosthetic limbs, exoskeletons, and humanoid robots lack critically needed capabilities.
Probably no other material has been described for so many fundamentally different types of actuators than carbon nanotubes. Demonstrated electrically powered and fuel powered nanotube actuators provide up to a few percent actuator stroke and a hundred times higher stress generation than natural muscle. Large stroke pneumatic nanotube actuators have been demonstrated that use electrochemical gas generation within nanotube sheets. In other studies, nanotubes have been used either as electrodes or as additives to profoundly modify the response of other actuating materials—like dielectric, ionically conducting, photoresponsive, shape memory, and liquid crystal polymers.
The following provide examples of these diverse types of actuators based on carbon nanotubes. Electrostatic attraction and repulsion between two nanotubes was used for cantilever-based nano-tweezers [P. Kim, C. M. Lieber, Science 126, 2148-2150 (1999)] and mechanically-based switches and logic elements [T. Rueckes, K. Kim, E. Joselevich, G. Y. Tseng, C.-L. Cheung, C. M. Lieber, Science 289, 94-97 (2000), V. V. Deshpande, H.-Y. Chiu, H. W. Ch. Postma, C. Mikó, L. Forró, M. Bockrath, Nano Letters 6, 1092-1095 (2006)]. On the macroscale, electrically powered [R. H. Baughman et al., Science 284, 1340-1344 (1999); U. Vohrer, I. Kolaric, M. H. Hague, S. Roth, U. Detlaff-Weglikowska, Carbon 42, 1159-1162 (2004); S. Gupta, M. Hughes, A. H. Windle, J. Robertson, J. Appl. Phys. 95, 2038-2042 (2004)] and fuel powered [V. H. Ebron et al., Science 311, 1580-1583 (2006)]carbon nanotube actuators provided up to a few percent actuator stroke and a hundred times higher stress generation than natural muscle. Demonstrated large stroke pneumatic nanotube actuators used electrochemical gas generation within nanotube sheets [G. M. Spinks et al., Advanced Materials 14, 1728-1732 (2002)]. Carbon nanotube composites with organic polymers provided photoresponsive [S. V. Ahir, E. M. Terentjev, Nature Materials 4, 491-495 (2005)], shape memory [H. Koerner, G. Price, N. A. Pearce, M. Alexander, R. A. Vaia, Nature Materials 3, 115-120 (2004), P. Miaudet et al., Science 318, 1294-1296 (2007)], and electromechanical [S. Courty, J. Mine, A. R. Tajbakhsh, E. M. Terentjev, Europhysics Letts. 64, 654-660 (2003)] actuators.
Major limitations exist for the above described carbon nanotube artificial muscles, as well as prior art artificial muscles of any type. These limitations include muscle stroke, stroke rate, cycle lifetime, or temperatures of operation—and in most cases a combination of some of these and other limitations (like energy conversion efficiency).
Embodiments of the present invention provide energy efficient artificial muscles that can operate at extreme temperatures (near 0 K to above 1900 K) where prior-art muscles cannot operate, stroke rates and strokes that can exceed 4×104%/s and 250% in one direction and generate over 30 times higher force than for the same weight and length natural muscle.
In addition to extending the capabilities of artificial muscles to giant strokes and strain rates at extreme temperatures, the actuator mechanism of the present invention enables applications that relate to structural changes during large-stroke actuation. These include, for example, the ability to dynamically modify the diffraction of light at over kilohertz frequencies for optical applications and the ability to tune the density of actuator sheets and then freeze this actuation for optimizing electrodes for organic light-emitting displays, solar cells, charge stripping from ion beams, and cold electron field emission. Prototypical actuator materials are provided that enhance actuation using nanoscale amplification effects, due to giant Poisson's ratios and even negative linear compressibilities. Embodiments of the present invention show that these giant Poisson's ratios can be used to amplify the strokes of other actuator materials and for such applications as sensors.